1. Field of the Invention
The present invention generally relates to a foil bearing which is one of the fluid bearings used in various machines and equipments and, more particularly, to the foil bearing suitable for supporting a rotary shaft of a kind used in machines and equipments requiring a high speed rotation of the rotary shaft, such as, in particular, a turbo machines including a turbo compressor, an expansion turbine and a gas turbine, and an electrostatic painting device of a rotary atomizing type, and also to a spindle device using the foil bearing and having the rotary shaft on which an atomizer head is mounted, for example, a high speed spindle device for use in a rotary atomizer for atomizing a liquid medium by the effect of a centrifugal force.
2. Description of the Prior Art
The rotary atomizer for atomizing, by the effect of a centrifugal force, a liquid medium supplied to a disk-shaped atomizer head then rotating at high speeds is generally known as used in an electrostatic painting machine of a rotary atomizer type for atomizing a painting material, a powder making machine, a spray drier and so on. The powder making machine is an apparatus in which a molten metal or the like is atomized under an atmosphere rich of an inert gas and is quickly cooled to provide atomized particles of the metal. The spray drier is used to manufacture a powder by atomizing a solution containing food materials or medicines in a hot blast to provide finely divided particles.
Hereinafter, reference will be made to the electrostatic painting machine of the rotary atomizer type for the discussion of prior art relevant to the present invention. In the electrostatic painting machine of the rotary atomizer type, while a rotary shaft having an atomizer head mounted on a free end thereof is driven at a high speed, a painting material is supplied to the atomizer head so that the painting material can be atomized by the effect of a centrifugal force. As bearings for rotatably supporting the rotary shaft, rolling bearings have hitherto been employed. However, the use of the rolling bearings have brought about problems associated with the lifetime which often decreased as a result of high speed rotation of the rotary shaft within an atmosphere rich of solvent gases and also with contamination of the painting material in contact with a lubricating oil leaking from the bearings. Accordingly, in order to alleviate those problems, air bearings have now come to be suggested and used in practice in place of the rolling bearings.
Some of the spindle devices used in the electrostatic painting machines utilizing the air bearings are shown in FIGS. 16 and 17. The spindle device shown in FIG. 16 is of a type in which externally pressurized air bearings that are supplied a compressed air from an external source are employed, and is disclosed in, for example, the Japanese Laid-open Patent Publication No. 9-173913. As shown therein, the spindle device includes a housing 561 having an axially extending internal bore 561a defined therein, in which a rotary shaft 567 and a turbine rotor 568 mounted on a rear end of the rotary shaft 567 for generating a rotational force are operatively accommodated through a bearing gap 566. A radial air bearing 569 is defined between the periphery of the rotary shaft 567 and air supply nozzles 564, while a thrust air bearing 570 is defined between the turbine rotor 568 and air supply nozzles 565. In this structure, when a compressed air is supplied into the bearing gap through the air supply nozzles 564 and 565, the rotary shaft 567 and the turbine rotor 568 are supported afloat by the effect of the pressure of the compressed air in a non-contact fashion relative to the housing 561 and, thus, externally pressurized air bearings including the radial air bearing 569 and the thrust air bearing 570 deploy their intrinsic function.
On the other hand, a plurality of turbine blades 579 are arranged on an outer peripheral face of the turbine rotor 568 on the rear end of the rotary shaft 567, while the housing 561 is formed with a compressed air supply port 580 that is communicated with an air compressor 581 for blowing the compressed air towards the turbine blades 579 in a direction substantially tangential to the turbine rotor 568. In this structure, when and so long as the compressed air is supplied from the compressor 581 to the turbine blades 579 through the compressed air supply port 580, the rotational force is applied to the turbine blades 579 and, hence, the rotary shaft 567 then supported afloat can be driven at a high speed.
A paint atomizer head 573 is mounted on a front end of the rotary shaft 567 for rotation together therewith and, hence, the painting material sprayed from the atomizer nozzle 577 is introduced towards an inner peripheral surface of a cup-shaped guide plate 574 through a discharge port 578 and is then atomized outwardly by the effect of a centrifugal force developed by a high speed rotation of the atomizer head 573. In such case, if the atomizer head 573 is electrostatically charged, the paint material when flowing in contact with the inner peripheral surface of the guide plate 574 can be charged to a negative charge and can then be deposited, by the effect of an electrostatic force, on an article to be painted that is electrically connected to the ground.
FIG. 17 illustrates the spindle device employing the radial air bearing employed in the form of a self-acting air journal bearing of a tilting pad type such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 56-163775. A cross-sectional view of such self-acting air journal bearing taken along the line VIII—VIII in FIG. 17 is shown in FIG. 18.
The spindle device shown in FIG. 17 includes front and rear housings 102 and 103 of a substantially hollow cylindrical configuration, which are connected together by means of bolts 104 in coaxial relation with each other. A rotary shaft 108 is rotatably inserted into the front housing 102 and a paint atomizer head 109 is mounted on a front end of the rotary shaft 108 by means of a nut 110 for rotation together with the rotary shaft 108. The front housing 102 is provided with two tilting pad air bearings 122 and 123 for rotatably supporting the rotary shaft 108. Each of the tilting pad air bearings 122 and 123 includes, as shown in FIG. 18 in a cross-sectional representation, three pads 124, 125 and 126 disposed around a hollow cylindrical body 108a of the rotary shaft 108 while being spaced a slight distance therefrom. Those pads 124 to 126 are supported by means of respective support pins 127, 128 and 129 for rocking motion. The support pin 127 is fixed in position through a support arm having a leaf spring 136a and, therefore, the pad 124 is urged towards the hollow cylindrical body 108a by the action of the leaf spring 136a. In this structure, when the rotary shaft 108 is driven, an ambient air can be drawn into respective gaps defined between the hollow cylindrical body 108a of the rotary shaft 108 and the pads 124 to 126 and, therefore, a pressure is developed by the effect of a so-called self-acting. Since the pads 124 to 126 can undergo rocking motion and since the pad 124 is resiliently supported for movement in a radial direction, any possible fluctuation of the rotary shaft 108 which would result from an unbalance and an aerodynamic instability can advantageously be suppressed to thereby enable the rotary shaft 108 to be stably supported even during a high speed rotation.
For rotatably supporting the rotary shaft 108 in an axial direction, the spindle device shown in FIG. 17 makes use of an externally pressurized air bearing. As shown in FIG. 17, a pair of disk-shaped runners 139 and 140 and a turbine wheel 142 are fixedly mounted on a shank portion 108c of the rotary shaft 108. An annular plate 144 formed with air discharge ports 151 and 152 for discharge of a compressed air is disposed between the runners 139 and 140 and spaced a slight distance therefrom, to thereby complete the externally pressurized air thrust bearing. In this structure, when the compressed air is jetted from a jet nozzle 157 towards the turbine wheel 142, the rotary shaft 108 can be driven by the compressed air so supplied.
Since a high voltage is applied in the case of the electrostatic painting machine, the spindle device is often driven by an air turbine in order to secure an electric insulation of the spindle. However, in the rotary atomized for use in other applications, an electric motor is generally used as a drive unit for driving the rotary shaft.
However, the Japanese Laid-open Patent Publication No. 56-163775 disclosing the spindle device shown in FIG. 17 merely describes that “a foil bearing can be suitably used in association with a rotary shaft that is driven at high speeds and, accordingly, foil bearings can be employed in place of the tilting pad type air bearings 122 and 123”, and makes no more mention of the details of the foil bearing.
The foil bearing referred to above is one of self-acting air bearings, in which a bearing surface is defined by a flexible or thin metal plate and the rotary shaft can be supported in a non-contact fashion by the action of a pressure developed between the metal plate and the rotary shaft by the effect of a self-acting brought about by rotation of the rotary shaft. Various types of the foil bearings have hitherto been suggested, some of which are shown in FIGS. 19A to 19C.
The foil bearing shown in FIG. 19A is of a structure in which a rotatable support roller 201 is disposed at three locations for applying a tension to an endless annular foil 203 so that when the rotary shaft 202 is in a stationary, i.e., halted condition, the outer periphery of the rotary shaft 202 is held in contact with the foil 203 at three positions each substantially intermediate between the neighboring support rollers 201. This type of the foil bearing is disclosed in, for example, the Japanese Laid-open Patent Publication No. 54-87343. According to this patent publication, it is described that since the foil revolves together with the rotary shaft before the revolution speeds of the rotary shaft increases to a value sufficient to develop a required pressure, neither friction nor frictional wear occur at the time of start and halt of the rotary shaft and, therefore, the foil bearing can have an increased lifetime.
The foil bearing shown in FIG. 19B includes a plurality of thin plates 303 cooperating to define the bearing surface and is so designed as to develop a pressure at a plurality of locations around the outer periphery of the rotary shaft (not shown) by the effect of a self-acting.
The foil bearing shown in FIG. 19c includes a top foil 405 encircling, in a generally single turn, the rotary shaft 401 through an annular gap A, and a bump foil is interposed between the housing 402 and the top foil 405 to support the latter.
Since the foil bearing is capable of tolerating a thermal deformation and a misalignment owing to its structure, the foil bearing is largely employed in turbo machines such as, for example, a gas turbine and a compressor.
While the rotary atomizer requires a frequent replacement of the atomizer head, a frictional wear of a mount used on the rotary shaft for supporting the atomizer head and/or a deformation of the atomizer head tend to bring about a considerable unbalance. Also, even an uneven deposition of a liquid medium to be treated and subsequent solidification thereof while being deposited also brings about a considerable unbalance. For these reasons, the spindle used in the rotary atomizer have to endure such a considerable unbalance while it rotates at a high speed. In addition, in order to treat a liquid medium of a high viscosity and/or to reduce the particle size of the atomized particles, the spindle has to be driven at a high speed. However, with the conventional spindle devices shown respectively in FIGS. 16 and 17, the spindle is unable to withstand against an exciting force resulting from the unbalance and the spindle will eventually fail to rotate as a result of contact of the rotary shaft with the bearing surface.
In the case of the electrostatic painting machine, it is a recent trend to mount the painting head, including the spindle, on an articulated robot to permit the latter to perform a spraying work. In such case, if the spindle is heavy, the articulated robot of a bulky size would be required, posing a problem associated with the space for installation and the cost. Because of this, it is necessary to reduce the weight of the spindle device. Since the bearing gap in the externally pressurized air bearing is determined by a difference in size of the various component parts, a highly accurate machining is required. For this reason, it is necessary to increase the rigidity of the component parts so that any undesirable deformation thereof during machining can be minimized. Also, it is necessary to provide an air supply passage for supplying a gaseous medium towards the bearing in the housing. By these reasons, it is difficult to reduce the wall thickness of the various component parts in an attempt to reduce the weight of the spindle device.
Other than the failure of the rotary shaft to rotate properly as a result of contact with the bearing surface, failure of the rotary shaft to rotate properly may occur when the liquid medium to be treated ingresses inside the spindle. With the conventional spindle device, once the failure occurs, the rotary shaft failing to rotate properly has to be removed from the spindle device and then to be submitted to the manufacturer for repair, resulting in both increase of the cost for repair and waste of a substantial amount of time. Where the line is desired to be operated while the rotary shaft is submitted to the manufacturer for repair, an extra spindle must be prepared for replacement with the rotary shaft and, therefore, maintenance is costly in this respect.
In the case of the spindle utilizing the tilting pad type bearings shown in FIG. 17, in order to secure a predetermined floating characteristic, the radius of curvature of the bearing surface defined by the bearing pads must be of a value greater by a few to 10 μm than the outer diameter of the rotary shaft and, in addition, since the bearing pads are of a generally arcuate shape, machining is more difficult to achieve than the externally pressurized bearing. Also, since the thrust bearing employed is in the form of the externally pressurized air bearing, respective positions of the pads of each radial bearing have to be carefully adjusted so that the runners 139 and 140 fixed on the rotary shaft can be disposed at a predetermined position with a slight thrust bearing gap defined in cooperation with the thrust bearing surface (opposite surfaces of the annular plate 144) fixed to the housing. Thus, machining and assembly adjustment are extremely difficult to achieve, making it difficult to achieve a mass-production and, therefore, this type of the spindle device has not yet been employed in the painting line.
As a bearing assembly for use in a machine of a kind requiring a high speed rotation, such a foil bearing as shown in FIG. 13 is also known in the art and is disclosed in, for example, the Japanese Laid-open Patent Publication No. 10-331846. FIG. 13 illustrates the foil bearing used as a journal bearing (a sliding bearing of a radial type), in which a bearing foil 53 disposed around the rotary shaft 54 with a bearing gap defined between it and the rotary shaft 54 is elastically supported by a bearing housing 51 through a multiplicity of bump foils 60 and 62. In this assembly, when the rotary shaft 54 rotates in a direction shown by the arrow 50, air can be drawn into a generally wedge-shaped gap defined between the rotary shaft 54 and the bearing foil 53 to develop a pressure by which a load capacity can be induced. In the event of a load acting on the rotary shaft 54, the bump foils 60 and 62 and the bearing foil 53 undergo deformation in dependence on change of a distribution of pressure within the bearing gap to properly correct the shape of the bearing gap (a gaseous film), thereby resulting in a stable operation. Also, by the action of a frictional force acting between the bump foils 60 and 62 and the bearing housing 51, also acting between the bearing foil 53 and the bump foils 60 and 62, the damping capacity can be deployed.
Although each of the bump foils 60 and 62 is prepared from a thin metal plate by corrugating it so as to have alternating hills and dales that continue axially and is therefore easily deformable in a plane perpendicular to a center axis, each bump foil is hard to deform in a plane parallel to the center axis. As such, deformation of the bearing foil 53 can results in an optimum shape in dependence on the distribution of pressure within the bearing gap so far as the circumferential direction is concerned, but so far as the axial direction is concerned the amount of deformation is substantially constant and the bearing gap cannot necessarily attain an optimum shape. In order to alleviate this drawback, in the example shown in FIG. 13, it is suggested to divide each of the bump foils 60 and 62 in an axial direction. An attempt is also made to superimpose the bump foils 60 and 62 of different shapes one above the other so that the spring characteristic thereof may have a non-linearity to thereby increase the load capacity and also to utilize a friction between the bump foils 60 and 62 to improve the damping characteristic. However, it has been found that these attempts tend to result in complication of the structure of the bump foils 60 and 62, accompanied by difficulty in manufacture and increase of the cost of manufacture.
Even in the foil bearing of the thrust type, the design similar to that shown in and described with reference to FIG. 13 has been suggested and is disclosed in, for example, the Japanese Laid-open Patent Publication No. 10-331847. This foil bearing of the thrust type is capable of supporting an axially acting load in a manner similar to that described with reference to FIG. 13.
The foil bearing of the type utilizing the bump foils as seen in any of the various bearings discussed above can have an increased load capacity if the shape of each of the bump foils is properly tailored so that a distribution of rigidity of support of the bearing foil 53 can be optimized. However, the structure tends to become complicated and compactization is hard to achieve and, yet, it tends to be expensive because a highly accurate press work is needed. Also, no sufficient damping capacity can be obtained in the event that the exciting force brought about by, for example, the unbalance of the rotating body is considerable.